Transformers for power supplies in computers and accessories must meet the demanding requirements of computing equipment for low losses in the switching mode and elimination of spikes in the output. Such losses typically result from leakage inductance and consequent leakage reactance caused by irregular spacing of the secondary windings relative to the primary windings. The leakage inductance is inversely related to mutual inductance coupling between the windings and results in a lower coefficient of coupling between the windings. Manufacturers generally require very low levels of leakage inductance. For example, the leakage inductance specification for a power supply transformer used with microprocessor computers and accessories is 45 microhenries (.mu.h) maximum.
This low specification for leakage inductance is difficult to achieve in toroid transformers with multiple secondary and tertiary and windings with intermediate insulating layers wound over the primary winding and toroidal core. The complex winding configurations must frequently be wound and placed by hand. The compound curvature of the core and the irregular surfaces produced by the complex of windings and insulating layers make it difficult to achieve regular and optimal spacing of the secondary windings relative to the primary winding for maximizing mutual inductance coupling.
Multifilar secondary windings are particularly prone to irregularity in the spacing of the multiple filaments on toroid transformers. The parallel filaments which comprise each turn of the multifilar secondary are used to achieve greater distribution and more even distribution of the secondary winding turns over the primary winding and therefore greater mutual coupling. However, irregularity and unevenness in the equal spacing of the parallel filaments of the multifilar secondary winding and crossover of the filaments interferes in the mutual inductance coupling and increases leakage inductance. Such irregular spacing and crossover of filaments is a particular problem on toroidal cores and in toroid transformers because of the irregular surfaces over which the multifilar secondaries must be wound, often by hand.